In an engine of an automobile or the like, as targeting improvement of fuel consumption due to improvement of engine warming-up performance and engine operation at appropriate temperature and the like, it has been considered to control an amount of cooling water to be cooled by a radiator owing to that a bypass passage to return cooling water directly to the engine as bypassing the radiator is arranged in addition to a main passage for circulating cooling water between the engine and the radiator and that an opening of a cooling water control valve which is arranged at the main passage is adjusted in accordance with cooling water temperature and other values. For example, at the time of engine starting having low cooling water temperature, engine warming-up is to be accelerated by closing the main passage and returning cooling water directly to the engine through the bypass passage without passing through the radiator. Further, for example, opening and closing (an opening degree) of the cooling water control valve is adjusted to control temperature of cooling water so as to improve fuel combustion of the engine after warming-up thereof.
For example, a rotary type valve which is driven with a stepping motor, a thermostat type valve which is operated with temperature and the like have been considered for such a cooling water control valve. Here, the thermostat type (thermosensitive type) valve denotes a valve having a thermostat, thermo-wax or the like to be displaced with temperature and being operated to be opened and closed with the displacement due to temperature.
Here, when the cooling water control valve becomes inactive in a closed state by any chance, cooling water is to be circulated in the engine through the bypass passage without being cooled by the radiator and cooling water temperature is increased. In this case, there is a possibility that the engine is overheated with continuous operation thereof. Accordingly, there has been proposed to circulate cooling water to a radiator side with a valve having a thermal safety device which is operated separately from a cooling water control valve when temperature of cooling water is increased owing to inactiveness or the like of the cooling water control valve in a closed state (see Patent Literature 1).
That is, the valve having the thermal safety device is a fail-safe mechanism. Here, the valve having the thermal safety device is a device using a thermally-displaced device formed of a thermostat, thermo-wax, shape-memory alloy, a combination of a spring and alloy to be melted at predetermined temperature, for example. When cooling water temperature is increased to the set temperature or higher, the device is displaced with temperature and the valve is to be in an opened state.
By the way, water jackets to circulate cooling water to an outer face side of an engine are arranged in the engine for cooling the engine. For example, a water jacket is arranged at a cylinder block of the engine and a water jacket is arranged at a cylinder head which is fixed to the cylinder block. In this case, cooling water is required to be circulated against the radiator for cooling the engine by causing cooling water to inflow to the water jacket at the cylinder block side and the water jacket at the cylinder head side and to outflow therefrom.
Here, during engine operation, improvement of fuel consumption of a vehicle is expected owing to that temperature of cooling water in the water jacket at the cylinder head side and temperature of cooling water in the water jacket at the cylinder block side are controlled to be different in temperature. Accordingly, in consideration of effectively cooling the cylinder head and the cylinder block as targeting improvement of fuel consumption, it is considered that circulation of cooling water is controlled separately. In this case, it is considered that cooling water control valves are arranged respectively to the water jacket at the cylinder head side and the water jacket at the cylinder block side, for example.